computation of cavity working dimension. Cavities are mold parts for forming the external shape of plastic parts. It’s working dimension is a kind of containment dimension which can gradually get larger due to the abrasion of cavity in use. Hence, to leave some space for mold-repair after abrasion and for the convenience of fitting and assembly, when designing mold, it might as well take the lower limit as the containment dimension and take the upper deviation as dimension tolerance.